Cytomel 25mcg Liothyronine is used to treat an underactive thyroid (hypothyroidism). It replaces or provides more thyroid hormone, which is normally made by the thyroid gland. Cytomel 25mcg Liothyronine is a man-made form of thyroid hormone. Low thyroid hormone levels can occur naturally or when the thyroid gland is injured by radiation/medications or removed by surgery.